This invention relates to a novel composition of matter comprising a highly reactive, highly amorphous, and anhydrous form of silicon oxide (silica); a method for its preparation and novel cement compositions prepared from such silica.
Certain agricultural organic materials are high in biogenetic silica, that is, silica occurring within the cell structure. Principally, rice hulls, rice stalks, equisetum, (a common weed popularly known as horsetails) and certain palm leaves, particularly palmyra palm, have varying amounts of silica in the cell structure. In addition, certain bamboo stems are known to contain relatively large amounts of silica and wheat straw contain from 2 to 3 percent silica in the cell structure. For example, most rice hulls are generally found to contain approximately 20% silica while rice straw may have approximately 181/2% silica. Certain California horsetail weeds are known to have about 20 to 25% silica.
The disposition of rice hulls and rice straw has become a substantial problem to the rice growing industry inasmuch as they apparently can be disposed of only by burning or burying. Dump space for burying the material has become scarce in the last few years and the incineration of the silica bearing hulls and straw in open fields generally creates undesirable atmospheric pollutants.
It has been known, of course, that silica, along with calcium oxides, is a component of portland cement, primarily occurring as complex calcium silicates. However, the silica produced by the incineration of silica containing organic agricultural materials can be used as a cement component only to the extent that it replaces sand or shale, because the incineration of the hulls inevitably leads to the production of ash containing crystalline forms of silica.
The phase diagram of silicon dioxide indicates that a transition from the amorphous, non-crystalline form of material to the crystalline forms known as tridymite and crystobalite takes place at very high temperatures when the silica is in pure state. Thus, with pure silica in the amorphous form, it is theoretically necessary to raise the temperatures above 2,000.degree. F to effectuate the transformation. However, the incineration of rice hulls, even at temperatures substantially below 2,000.degree. F, has always lead to the formation of crystalline varieties of silica, because the transition temperature from amorphous to crystalline is reduced substantially by the presence of other components of the original rice hulls.
Prior attempts to use crystalline silica as an active component of portland cement have always involved heat treating mixtures of limestone and siliceous shales or clays at temperatures in excess of 2600.degree. F. The same objective can also be accomplished by application of mechanical energy. Experimentally, it has been shown that attrition grinding of crystalline quartz can activate the silica by rupturing the chemical bonds at the surface. In addition to attrition grinding, vibromilled sand-lime mixtures which are attrition ground, have been reported to have acquired binding properties, and thus treated lime-sand mixtures have been used as hydraulic cements.
These processes, except for the commercial process of heating a siliceous raw material with limestone, appear to be commercially unattractive because of the excessive mechanical energy necessary in the vibromilling and attrition grinding processes.